1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit (IC) manufacturing, and more particularly, to a modified LOCOS isolation process for preventing the bird's beak effect, in which oxygen ions are implanted with a tilt angle into a semiconductor substrate and a thermal oxidation process is then performed to form a field oxide layer having substantially uniform thickness.
2. Description of Related Art
With the continual improvement of semiconductor integrated circuit fabrication techniques, the number of devices that can be packed onto a semiconductor chip has increased greatly, while the geometric dimensions of the individual device have been markedly reduced. In today's fabricating process, the feature size has shrunk into the sub-micron range. In such high-density chips, each of the elements must be isolated properly in order to obtain good electrical characteristics. Device isolation technology has been developed to fulfill the above requirement, the main purpose of which is to provide good insulation between the elements using a smaller isolation area so that there is additional space for building more elements.
In the past, a so-called LOCal Oxidation of Silicon (LOCOS) technique has been widely used for device isolation of the integrated circuit chips. According to this method, a thick oxide layer is grown as an isolating region. To easily understand the technique, please refer to FIGS. 1A and 1B, which illustrate a prior-art LOCOS process. As shown in FIG. 1A, there is a semiconductor substrate 10, such as a silicon wafer. A stacked shielding layer including a pad oxide layer 11 and a silicon nitride layer 12 is formed on the semiconductor substrate 10. Appropriate photolithography and etching processes are then performed to define an opening 13 in the pad oxide layer 11 and the silicon nitride layer 12, so as to expose the area for forming a device isolating region.
Referring next to FIG. 1B, a thermal oxidation process is carried s out. For example, the silicon wafer is placed in a furnace and the temperature is raised to somewhere between 800 to 1100.degree. C. An oxygen-containing gas is then introduced into the furnace to start the thermal oxidation for growing a thick field oxide 14 to define the margin of an active area. Since the oxidizing speed of silicon nitride is less than that of silicon, the silicon nitride layer 12 works like a mask against thermal oxidization, so the field oxide 14 grows only where the substrate 10 is not covered by the silicon nitride layer 12. After that, the silicon nitride layer 12 and the pad oxide layer 11 are removed to complete the LOCOS isolation process.
Although the conventional LOCOS technique can provide feasible isolation between elements of the integrated circuit by a relative simple process, there are a number of disadvantages becoming rather unacceptable when we attempt to apply this technique in fabricating sub-micron devices. First, the oxidization of silicon happens not only in the vertical direction but also in the horizontal direction. As a result, a part of the field oxide 14 grows beneath the adjacent silicon nitride layer 12 and lifts it up. This is termed the "bird's beak effect" by persons skilled in the art. Secondly, due to the stresses caused by the bird's beak effect, a part of nitrogen in the compressed regions of silicon nitride layer 12 diffuses to adjacent tensile strained regions at the interface of the pad oxide layer 11 and the substrate 10, and forms a silicon-nitride-like layer 15. In subsequent process steps of forming gate oxides (not shown), due to the masking effect of the silicon-nitride-like layer 15, the gate oxides will be thinner than they should be. It is termed the "white ribbon effect" because a white ribbon will appear at the edges of the active regions under examination by optical microscopes.
Additionally, because the volume of silicon dioxide is 2.2 times that of silicon, the field oxide 14 protrudes above the surface of the silicon substrate 10, forming a non-recessed surface. Also, due to the lateral expansion of the field oxide 14 during oxidation, a great deal of stress occurs in the active region. Many crystalline defects are produced near the bird's beak regions, which result in an increase of junction leakage and in a reduction of the reliability of the devices. Furthermore, the oxygen diffusion rate through the narrow opening differs from that through the wide opening. Thus, after the thermal oxidation process, the thickness of the field oxide within a narrow opening is thinner than that of the field oxide within a wide opening. This also alters the isolation property that should be.
To overcome the above-mentioned disadvantages of the conventional LOCOS process, many modified methods of forming device isolation regions are disclosed. For example, U.S. Pat. No. 4,211,582 to Horng et al uses a two-step oxidation process with plurality masks technique, and U.S. Pat. No. 4,868,136 to Ravaglia describe a combination of LOCOS and "trench" techniques. However, in each of these above processes there are some inherent drawbacks is such areas as productivity, repeatability, complexity, and maturity, as well as global planarization.